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Junior School: Chemical Changes

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CHEMICAL CHANGES

 

Chemical Change of Candle - JavaLab

chemical change happens when one chemical substance is transformed into one or more different substances, such as when iron becomes rust. Chemical changes occur through the process of chemical reactions, and the resulting substances have different properties because their atoms and molecules are arranged differently.

CHEMICAL CHANGE V PHYSICAL CHANGE

Chemical and Physical Changes of Matter

Chemical Changes

A chemical change involves a chemical reaction to produce a new product. It is a change at the molecular level of matter. Chemical bonds between atoms break and then form to connect different atoms.

Examples of Chemical Changes

In a chemical change, new product forms as atoms rearrange themselves. Chemical bonds are broken and reform to make new molecules. Examples of chemical changes include:

  • Souring milk
  • Digesting food
  • Cooking an egg
  • Baking a cake
  • Rusting iron
  • Mixing an acid and a base
  • Burning a candle
  • Mixing baking soda and vinegar

Physical Changes

A physical change is a change in matter that alters its form but not its chemical identity. The size or shape of matter often changes, but there is no chemical reaction. Phase changes are physical changes. These include melting, boiling, vaporization, freezing, sublimation and deposition. Breaking, crumpling, or molding matter also results in a physical change. Many physical changes are reversible.

Examples of Physical Changes

Examples of physical changes include:

  • Melting an ice cube
  • Freezing an egg
  • Boiling water
  • Sublimation of dry ice into carbon dioxide gas
  • Shredding paper
  • Crushing a can
  • Breaking a bottle
  • Chopping vegetables
  • Mixing sand and salt
  • Making sugar crystals
  • Dissolving sugar in water (the sugar mixes with the water, but can be recovered by evaporation or boiling)

HOW TO TELL CHEMICAL AND PHYSICAL CHANGES APART

Matter: Physical and Chemical Changes PowerPoint and Notes by Teaching Works

How to Tell Chemical and Physical Changes Apart

The key to distinguishing between chemical and physical changes is determining whether there is a new substance that wasn’t there before. If you see signs of a chemical reaction, it’s probably a chemical change. Signs of a reaction include:

  • Temperature change
  • Light
  • Color change
  • Bubbling
  • Odor
  • Sound
  • Formation of a precipitate

If none of these signs are present, it’s a good bet a physical change occurred.

Some people use reversibility as a test for chemical and physical changes. The premise is that a physical change can be undone, while a chemical change can only be reversed by another chemical reaction. This is not a great test because there are too many exceptions. While you can melt and freeze an ice cube (a physical change), it’s much harder to reassemble shredded paper (another physical change).

Most physical changes can be reversed if energy is added. Chemical changes can only be reversed via another chemical reaction.

CHEMICAL REACTIONS

Examples of Chemical Reactions in Everyday Life

Examples of Chemical Changes in Everyday Life

Chemical changes might bring to mind fireworks or explosions. However, some examples of chemical changes are subtle. Explore examples of chemical changes in cooking, nature, and your home. Get an understanding of the difference between physical and chemical changes.

Examples of Chemical Changes

Not all reactions are chemical. However, if a reaction makes a new substance, then you are looking at a chemical change. For example, those fireworks covering the sky on the fourth of July are the result of a chemical change. Photosynthesis in plants is also an example of a chemical reaction. In this case, sunlight is changed into food for the plant. Typically, chemical reactions have a few tells, including:

  • Colour changes
  • Releasing or absorbing of energy
  • Odour changes
  • Gas production

Chemical changes happen all around you, even in your own home. Explore some fun examples in cooking, nature, and your home.

Examples of Chemical Changes in Cooking

When you mix the flour and sugar to make a cake, you might not think it's a chemical reaction at play but it is. Baking a cake is a classic example where a new substance is created from mixing flour, sugar, vanilla, cocoa, and heat. Here are a few other cooking examples.

Rising Bread

Bread rising is exciting, right? Not so much! However, not all chemical changes are exciting. While not thrilling, the yeast causing the bread to rise is a chemical change. Why? Well, yeast breaks the starches in the bread dough down into sugars, and those sugars metabolize into carbon dioxide and ethanol. The carbon dioxide makes the bread rise.

Caramelization

Do you like caramelized onions? Either way, it’s a great example of a chemical change. In caramelization, the sugar is broken down to create a specific aroma and taste.

Maillard Browning

Maillard browning is a strange name, but it's how you get that delectable brown crust on your bread and barbecued meat. Maillard browning, named for the chemist Louis-Camille Maillard, is a series of chemical reactions starting with amino acids and sugar.

Natural Examples of Chemical Changes

Cooking isn’t the only place you see chemical changes. From the way that your body digests food to a match burning when you strike it, chemical reactions can be found everywhere. Dive into a few different examples in nature and your home.

Leaves Changing Colour

Those beautiful colours of fall aren’t happening by chance. They are thanks to chemical changes in the leaves. Typically, leaves are green due to chlorophyll used for photosynthesis. However, before trees take their winter nap, chlorophyll in the leaves breaks down. As the chlorophyll breaks down, the carotene and anthocyanins naturally in leaves all summer long take centre stage creating those beautiful red and yellow colours.

Souring Food

One of the key characteristics a chemical change has taken place is odour, which is why people might smell foods to see if they went bad. For example, milk goes bad because of fermentation. This is where the lactose is transformed into lactic acid by bacteria surviving pasteurization.

Combustion

When something undergoes a chemical reaction, it becomes something else. For example, when wood burns, it becomes ash. The reason wood burns is due to combustion.

Mixing Baking Soda and Vinegar

If you mix an acid, such as vinegar, with a base, like baking soda, you’ll get a chemical reaction. In this case, it starts to bubble. This is a great way to create a homemade volcano.

Rusting

You never want things to rust. However, no matter where you live, it’s inevitable. Rust is the chemical change taking place when iron is exposed to metal. Unless your car is made of plastic or in a bubble, it will find a way to rust.

REVERSIBLE AND IRREVERSIBLE CHANGE

 

Reversible and irreversible reactions - YouTube

At the start of the 1800s, scientists thought reactions only went in one direction, which seems reasonable. After all, for a lot of processes, you can’t get things to react in the opposite direction. Imagine trying to uncombust a combustion reaction, like when methane burns in oxygen to form carbon dioxide and water. We’d have to force carbon dioxide and water to react to form methane and oxygen again. Impossible! But there was a French chemist named Claude Louis Berthollet that was studying a reaction where sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride.

He took a trip to a salt lake where he made an interesting observation. There was sodium carbonate forming at the edge of the lake. He eventually realized that the large amount of salt in the salt lake or sodium chloride was reacting with the calcium carbonate that’s found in the shells of marine organisms. In other words, the reaction was going in the reverse direction of what he was studying in the lab. As it turns out, there’s many reactions that don’t just go in the forward direction where the reactants combine to form the products. There’s many reactions that can go in the reverse direction as well, where the products can combine to form the reactants.